Thank you for that information. I understand you are upset but I don't need you to repeat the facts that you have already posted. It just takes more time for you and me to go over them again. In any event, the police didn't have to arrest anybody regardless of what they say unless they thought there was probable cause to believe there was an act of violence versus an accident. So, they apparently did not believe you for whatever reason. At this point, because the State decides what cases to prosecute or not, you have no real control over whether or not they decide to formally charge your roommate.
However, if he is charged by the State you can make your case for the facts as you are telling them to the prosecutor when they interview you or question you at trial, should this go that far. You cannot, however, refuse to testify. That would get you arrested for contempt of court. You can tell the truth and then it is up to the Court to decide whether or not there are enough facts to prove a case beyond a reasonable doubt, assuming your friend pleads not guilty.
All that said, it is unlikely that, unless there is very strong evidence of violence here, that the State will even go forward with this case if you are not willing to say he was violent toward you. That is because these cases are very difficult to prove without the corroborating testimony of the alleged victim unless, again, there is very obvious evidence of violence (more than a bruise) or there is a pattern with this alleged offender.
His attorney's job is to get the charges dismissed or a not guilty finding. You can't do that, his attorney will have to. They might ask you to testify on his behalf if it comes to that. But again, the prosecutor isn't likely to go forward with an actual charge unless they think they can prove violence without your testimony.
Please feel free to ask for clarification if needed. If none is needed, then if you could take a moment to leave a positive rating in the box above, I will receive credit for assisting you today. Thank you Amberlee